1. Field of the Invention
The present invention relates to a new electric discharging power supply which is capable of permitting stable electric discharge to appear effectively in plasma discharge appliances.
2. Description of the Related Art
Generally speaking, the method of producing electric discharge includes four different electric discharges, that is, dc or ac electric discharge, high-frequency, static-electricity coupling electric discharge, high-frequency, inductive magnetic coupling electric discharge and microwave electric discharge.
The feature of each electric discharge is described below in terms of discharging condition, power supply structure and other factors.
The feature of dc electric discharge is: advantageously plasma density, plasma temperature and other plasma parameters are stable with time, and no load matching is required. Accordingly the structure of power supply can be simplified. Disadvantageously accidental adhesion of undesired dielectric substance to the electrode surface causes disappearance of electric discharge. Electric discharge cannot appear easily at a relatively low gas pressure.
The feature of ac electric discharge is: advantageously required oscillation and amplification of applied voltage can be easily attained because of its low frequency, and the load matching can be easily attained, too. Particularly electric discharging power supplies of commercial frequency (50 or 60 Hz) can be composed of transformers only, and therefore, power supplies of increased capacity can be easily built up.
Disadvantageously plasma parameters, however, are liable to vary with the frequency of power supply, and accidental adhesion of undesired dielectric substance to the electrode surface causes disappearance of electric discharge. Electric discharge cannot appear easily at a relatively low gas pressure, as is the case with the dc electric discharge.
The common feature both of high-frequency, static-electricity coupling electric discharge and high-frequency, inductive magnetic coupling electric discharge is: electric discharge cannot be adversely influenced by undesired dielectric substance even if accidentally adhered to the electrode surface, and the electric discharge remains stable. Electric discharge can be initiated even at a relatively low gas pressure.
Disadvantageously plasma parameters, however, are liable to vary with the frequency of power supply. Required oscillation-and-amplification and the load matching cost much because of the high frequency used, and plasma cannot be evenly expanded in a relatively large space without difficulty.
The feature of microwave electric discharge is: it uses such a high frequency that no electrode may be required; plasma parameters are almost stable with time as is the case of dc electric discharge; and electric discahrge can be produced with ease at a relatively low gas pressure. Disadvantageously use of extremely high frequency requires much cost involved for oscillation, amplification, power transmission, load matching and other equipment matters; and the even expansion of plasma in a relatively large space cannot be attained with ease.
In case of ac electric discharge and high-frequency electric discharge both using electrodes the voltage applied between two electrodes will vary sinusoidally, and therefore, the applied voltage will be zero once for each period, and therefore, the electric discharge will be interrupted once for each period. Therefore, if a plasma resulting from such an electric discharge has a life shorter than the period of alternating voltage, the plasma varies largely with the frequency of applied voltage.
As may be realized from the above, the structure of power supply can be simplified as the frequency used is reduced. Use of ac or dc power supplies permits an increased amount of electric discharge to appear easily at a relatively low cost, but electric discharges are liable to disappear in the presence of undesired dielectric substance accidentally adhered to the electrode surface or in the atmosphere of relatively low pressure. In case of ac power supplies plasma parameters are changeable.
Power supplies of increased frequency are free of such defects, but very expensive and not satisfactory in producing electric discharge in a relatively large space.
One object of the present invention is to provide a new ac electric discharging power supply using a plurality of phase-controlled power supply units and associated electrodes to produce electric discharge of relatively low frequency including commercial frequencies, which electric discharge is equivalent to the one produced at high frequencies, not being influenced adversely by undesired dielectric substance accidentally adhered to the electrode surface or by lowering the gas pressure, still assuring that plasma parameters are almost stable with time.
The power supply uses such a low frequency that a large amount of electric discharge may be permitted at a relatively low cost, and this feature is very advantageous to reduction of the invention to practice.